This invention pertains to isomerization processes and, more particularly, to processes whereby xcex3,xcex4-epoxy-alkenes and xcex3,xcex4-epoxycycloalkenes are isomerized to obtain the corresponding 2,5-dihydrofuran compounds. This invention also pertains to novel catalyst systems useful in the described isomerization processes and to methods for the preparation of supported catalyst systems.
Dihydrofurans are reactive heterocyclic species which are useful in a variety of applications, e.g., as intermediates in the production of useful polymers and chemicals. However, the use of dihydrofurans for such purposes has heretofore been restricted due to the non-availability of cost-effective preparative procedures therefor.
In addition, dihydrofurans are readily reduced to produce the corresponding tetrahydrofuran species, which are also useful in a variety of applications, e.g., as polar aprotic reaction solvents, co-solvents, reactive intermediates in the production of useful polymers, copolymers, and the like.
U.S. Pat. Nos. 3,932,468 and 3,996,248 disclose the production of 2,5-dihydrofurans by the rearrangement of substituted or unsubstituted epoxyalkenes with a homogeneous catalyst system comprising hydrogen iodide or hydrogen bromide and a transition metal Lewis acid in an organic solvent. This process suffers from a number of disadvantages including the use of corrosive hydrogen halides, the need for expensive, high-boiling tertiary amide solvents, e.g., N-methyl-2-pyrrolidinone, to dissolve the transition metal Lewis acid. We have found that the process of U.S. Pat. Nos. 3,932,468 and 3,996,248 also results in the unwanted production of up to 15% xcex1,xcex2-unsaturated aldehydes or ketones.
The thermal (i.e., non-catalytic) rearrangement of 3,4-epoxy-1-butene has been studied and shown by Crawford et al in the Canadian Journal of Chemistry, Vol. 54, pages 3364-3376 (1976) to produce a variety of products, including 2,3-dihydrofuran, cis and trans 2-butenal and 3-butenal.
Other reactions of epoxides have been reported. See, for example, U.S. Pat. No. 4,600,800, where epoxides are converted to allylic alcohols by contacting an epoxide in the liquid phase with solid alumina catalysts.
Another example of the rearrangement of epoxides is described in the Journal of Organometallic Chemistry, Vol. 359, pages 255-266 (1989), wherein Sato et al report the formation of xcex1,xcex2-unsaturated aldehydes and ketones by the rhodium (I) catalyzed isomerization of 1,3-diene monoepoxides.
U.S. Pat. No. 4,897,498 describes an efficient process for the preparation of xcex3,xcex4-epoxyalkenes by the selective monoepoxidation of dienes. Thus, a process is needed for the conversion of such epoxyalkenes to dihydrofurans in satisfactory selectivity and/or yields wherein the product may be readily recovered from the catalyst and the catalyst reused and used in continuous operation.
In accordance with the present invention, we have discovered a catalytic process for the isomerization of xcex3,xcex4-epoxyalkenes to produce dihydrofurans. The process provides high levels of epoxyalkene conversion with high selectivity to the desired dihydrofuran product. Long catalyst lifetimes are realized and the product may be recovered by relatively simple means since the catalyst and reaction mixture are readily separated by such simple techniques as distillation, decantation, filtration, gas stripping methods, gas/liquid flow separation, and the like.
Our invention also provides novel catalyst systems, both supported and unsupported, which are useful, for example, to promote the isomerization of epoxyalkenes to dihydrofurans. Processes for preparing the supported catalyst systems are also provided herein.
In accordance with the present invention, there is provided a process for the isomerization of xcex3,xcex4-epoxyalkenes to the corresponding 2,5-dihydrofuran compounds, which process comprises contacting a xcex3,xcex4-epoxyalkene or xcex3,xcex4-epoxycycloalkene with a catalytic amount of a quaternary organic onium iodide, e.g., a compound consisting of an ammonium, phosphonium or arsonium cation and an iodide anion, under isomerization conditions of temperature and pressure.
The xcex3,xcex4-epoxyalkene and xcex3,xcex4-epoxycycloalkene reactants may contain from 4 to about 20 carbon atoms, preferably from 4 to about 8 carbon atoms. Examples of the epoxyalkene and epoxycycloalkene reactants include compounds having the structural formula: 
wherein each R1 is independently selected from hydrogen, alkyl of up to about 8 carbon atoms, a carbocyclic or heterocyclic aryl group of about 5 to 10 carbon atoms or halogen or any two R1 substituents collectively may represent an alkylene group forming a ring, e.g., alkylene containing in the main chain up to about 8 carbon atoms. The preferred epoxyalkene reactants comprise compounds of formula (I) wherein only two of the R1 substituents individually may represent lower alkyl, e.g., alkyl of up to about 8 carbon atoms, or collectively represent straight or branched chain alkylene of up to about 8 carbon atoms. Exemplary compounds contemplated for use in the practice of the present invention include 3,4-epoxy-3-methyl-1-butene, 2,3-dimethyl-3,4-epoxy-1-butene, 3,4-epoxycyclooctene, 3,4-epoxy-1-butene, 2,5-dimethyl-2,4-hexadiene mono-epoxide, and the like. The epoxyalkene reactant of primary interest is 3,4-epoxy-1-butene.
The 2,5-dihydrofuran compounds obtained in accordance with our novel process have the structural formula: 
wherein the R1 substituents are defined above. Of the compounds which may be obtained in accordance with our invention, the most important is 2,5-dihydrofuran.
The quaternary onium iodide compounds which may be used as the catalyst in our novel process are known compounds and/or may be prepared according to published procedures. See, for example, U.S. Pat. No. 3,992,432 and the references cited therein. Exemplary quaternary organic onium iodide compounds include mono-, di-, tri-, or tetra-substituted quaternary onium iodides, wherein said substituents are selected from hydrogen, alkyl or substituted alkyl groups, cycloalkyl or substituted cycloalkyl groups, carbocyclic aryl or substituted carbocyclic aryl groups, heteroaryl or substituted heteroaryl groups, ferrocenyl, wherein each of said substituents may be bonded to one another to form a cyclic, heterocyclic, polycyclic or poly-heterocyclic structure. When used on a support or as a melt, the onium compounds normally contain at least 6 carbon atoms, preferably at least 12 carbon atoms, and have melting points not greater than about 225xc2x0 C., preferably not greater than about 200xc2x0 C.
Examples of the onium iodide catalysts are compounds conforming to the formulas
(R2)4Y+ Ixe2x88x92, xe2x80x83xe2x80x83(III) 
Ixe2x88x92 (R2)3Y+xe2x88x92R3xe2x88x92Y+x(R2)2+x Ixxe2x88x92, xe2x80x83xe2x80x83(IV) 

wherein
each R2 independently is selected from hydrogen, alkyl or substituted alkyl moieties having up to about 20 carbon atoms, cycloalkyl or substituted cycloalkyl having about 5 to 20 carbon atoms, or aryl or substituted aryl having about 6 to 20 carbon atoms; or when Y is P, each R2 also may be selected from alkoxy of up to about 20 carbon atoms, cycloalkoxy of about 5 to 20 carbon atoms, aryloxy of 6 to 10 carbon atoms or halogen;
two or three R2 substituents collectively may represent joined hydrocarbylene groups, e.g. alkylene having 4 to 6 main chain carbon atoms or unsaturated groups such as xe2x80x94CHxe2x95x90CHCHxe2x95x90CHCHxe2x95x90 and lower alkyl substituted alkylene and unsaturated groups, which form a mono- or poly-cyclic ring with the Y atom to which they are bonded;
each R3 is independently selected from hydrocarbylene moieties or substituted hydrocarbylene moieties;
x is 0 or 1, and
Y is N, P or As; provided that the quaternary onium iodide compound contains at least 6 carbon atoms.
The substituted groups and moieties referred to above bear one or more substituents such as groups having the formulas
xe2x80x94OR4, 
xe2x80x94Si(R4)3 and X 
wherein each R4 is independently selected from hydrogen or alkyl of up to about 20 carbon atoms and X is halogen. As used herein, the terms xe2x80x9chydrocarbylene moietiesxe2x80x9d refers to alkylene moieties having up to about 6 carbon atoms, arylene or polyarylene moieties having 6 to 20 carbon atoms.
The preferred onium iodide catalysts are the quaternary ammonium and quaternary phosphonium iodide compounds. Exemplary ammonium compounds include tetrapentylammonium iodide, tetrahexylammonium iodide, tetraoctylammonium iodide, tetradecylammonium iodide, tetradodecylammonium iodide, tetrapropylammonium iodide, tetrabutylammonium iodide, monooctylammonium iodide, dioctylammonium iodide, trioctylammonium iodide, N-octylquinuclidinium iodide, N,Nxe2x80x2-dimethyl-N,Nxe2x80x2-dihexadecylpiperazinium diiodide, dimethyl-hexadecyl-[3-pyrrolidinylpropyl]ammonium iodide, N,N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x2-hexa(dodecyl)octane-1,8-diammonium diiodide, N,N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x2-hexa(dodecyl)butane-1,4-diammonium diiodide, N-octylpyridinium iodide, and the like.
Exemplary phosphonium compounds include tetra-octylphosphonium iodide, tetrabutylphosphonium iodide, triphenyl(hexyl)phosphonium iodide, triphenyl(octyl)-phosphonium iodide, tribenzyl(octyl)phosphonium iodide, tribenzyl(dodecyl)phosphonium iodide, triphenyl(decyl)-phosphonium iodide, triphenyl(dodecyl)phosphonium iodide, tetrakis(2-methylpropyl)phosphonium iodide, tris(2-methylpropyl)(butyl)phosphonium iodide, triphenyl(3,3-dimethylbutyl)phosphonium iodide, triphenyl(3-methylbutyl)phosphonium iodide, tris(2-methylbutyl)(3-methylbutyl)phosphonium iodide, triphenyl[2-trimethylsilylethyl]phosphonium iodide, tris(p-chlorophenyl)(dodecyl)phosphonium iodide, hexyl-tris(2,4,6-trimethylphenyl)phosphonium iodide, tetradecyltris(2,4,6-trimethylphenyl)phosphonium iodide, dodecyltris(2,4,6-trimethylphenyl)phosphonium iodide, and the like.
Tetra-substituted ammonium and phosphonium iodide compounds containing a total of about 16 to 60 carbon atoms are especially preferred. Such compounds have the formulas 
wherein
each R5 substituent independently is selected from alkyl of up to about 20 carbon atoms and each R6 substituent is independently selected from R5, benzyl, phenyl or phenyl substituted with up to 3 substituents selected from lower alkyl (alkyl of up to about 4 carbon atoms) lower alkoxy or halogen; or
two R5 substituents collectively may represent alkylene of 4 to 6 carbon atoms including alkylene of 4 to 6 carbon atoms substituted with lower alkyl; provided, as specified above, that the quaternary iodide compounds contain about 16 to 60 carbon atoms.
Another group of preferred ammonium iodide compounds are comprised of N-alkyl-azabicycloalkane and N-alkyl- and N,Nxe2x80x2-dialkyl-diazabicycloalkane iodide compounds containing 6 to about 12 ring carbon atoms, e.g., bicyclic compounds having the general formula 
wherein R5 is defined above and A is the residue of an azabicycloalkane or diazabicycloalkane having 6 to 12 ring carbon atoms (including the 2 carbon atoms in the above general formula), e.g., azabicyclooctane, azabicyclononane, diazabicyclooctane and the like.
The onium iodide compounds described hereinabove may be employed in combination with a Lewis acid to catalyze the isomerization process of our invention. Examples of such optional Lewis acid co-catalysts include the alkali metal halides, zinc halides, magnesium halides, tin (II) halides, tin (IV) halides, titanium (IV) halides, titanium (IV) tetra-lower-alkoxides, zirconium (IV) halides, manganese (II) halides, iron (III) halides, or iron (III) acetylacetonate. Preferably, the Lewis acid co-catalyst is an alkali metal iodide, zinc iodide, zinc chloride, magnesium iodide, tin (II) iodide, tin (IV) iodide, titanium (IV) iodide, titanium (IV) tetramethoxide, titanium (IV) tetraethoxide, titanium (IV) tetraiso-propoxide, zirconium (IV) iodide, manganese (II) iodide, manganese (II) chloride, iron (III) iodide, iron (III) acetylacetonate or a combination thereof. The Lewis acid co-catalysts which are particularly preferred are polarizable iodides, such as, for example, titanium (IV) iodide, zirconium (IV) iodide, and, especially, zinc iodide and tin (II) iodide.
The Lewis acid co-catalyst alternatively may be selected from organotin (IV) and organoantimony (V) compounds such as hydrocarbyltin trihalides, dihydro-carbyltin dihalides, trihydrocarbyltin halides, tetra-hydrocarbyltin compounds and tetrahydrocarbylantimony halides. Examples of such organometallic compounds include compounds having the formula
(R7)nxe2x80x94Snxe2x80x94Hal(4xe2x88x92n) and (R7)4xe2x80x94Sbxe2x80x94Hal
wherein
each R7 independently is selected from alkyl or substituted alkyl moieties having up to about 20 carbon atoms, cycloalkyl or substituted cycloalkyl having about 5 to 20 carbon atoms, carbocyclic aryl or substituted carbocyclic aryl having about 6 to 20 carbon atoms, or heteroaryl or substituted heteroaryl moieties having about 4 up to 20 carbon atoms;
Hal is a halogen atom such as bromo or, preferably, iodo; and
n is 1, 2, 3 or 4.
Examples of organometallic compounds include dibutyltin diiodide, tributyltin iodide, trioctyltin iodide, triphenyltin iodide, tributyltin bromide, trimethyltin iodide, butyltin triiodide, tetrabutyltin, tetraoctyltin, triphenyltin iodide, tribenzyltin iodide, dimethyltin diiodide, diphenyltin diiodide, triphenyltin bromide and tetraphenylantimony iodide.
The preferred organometallic compounds comprise tin (IV) iodides having the above general formula and a total carbon content of about 3 to 24 carbon atoms wherein
each R7 substituent independently is selected from alkyl of up to about 12 carbon atoms, benzyl, phenyl or phenyl substituted with up to 3 substituents selected from lower alkyl, lower alkoxy or halogen;
Hal is iodo; and
n is 2 or 3.
The quaternary organic onium iodide catalyst, or the quaternary organic onium iodide-Lewis acid catalyst system, may be employed in the process provided by this invention in either a supported or unsupported form. The supported catalysts of this invention comprise an essentially non-acidic catalyst support material having one or more quaternary organic onium iodide compounds distributed on the surface thereof as a substantially continuous and uniform film and, optionally, one or more of the Lewis acids described above, e.g., an alkali metal halide, zinc halide, magnesium halide, tin (II) halide, tin (IV) halide, titanium (IV) halide, titanium (IV) lower alkyl alkoxide, organotitanium (IV) halide, zirconium (IV) halide, manganese (II) halide, iron (III) halide, iron (III) acetylacetonate or one of the organotin compounds or organoantimony halides described hereinabove.
The essentially non-acidic support may be in the form of a powder or shaped material having sufficient structural integrity to allow passage of gaseous reactant through a packed or fluidized bed of the supported catalyst under reaction conditions. Preferred support materials employed in the practice of the present invention are materials having a particle size in the range of about 20 up to 200 microns and having a crush strength of at least about two pounds. Support materials having crush strengths of at least ten pounds are especially preferred.
A variety of shapes are suitable for use as the support material employed in the practice of the present invention. For example, pellets, spheres, rings, saddles, extruded cylinders, and the like can be employed, so long as such materials have dimensions and packing characteristics so as to allow for the ready passage of gaseous reactant and product through a packed or fluidized bed of the catalyst under reaction conditions.
Examples of the materials which may be employed as the support include zinc oxide, zinc carbonate, magnesium oxide, silica, alumina, titanium oxide, lanthanum oxide, boron nitride, boron carbide, silicon nitride, silicon carbide, tin oxide, calcium oxide, barium oxide, strontium oxide, zirconium oxide, carbon, boron phosphate, or zirconium phosphate, as well as mixtures of any two or more thereof. The preferred support materials contemplated for use in the practice of the present invention include zinc oxide, zinc carbonate, magnesium oxide, silica, alumina, titanium oxide, boron nitride, silicon nitride, silicon carbide, calcium oxide, barium oxide and carbon as well as mixtures of any two or more thereof. Silica, alumina, titanium oxide and zinc oxide are particularly preferred support materials.
The amount of the quaternary organic onium iodide component of the novel catalyst compositions of this invention can vary substantially depending, for example, on the particular support material and the form, e.g., surface area, thereof, the mode in which the isomerization process is operated, the particular quaternary onium iodide present, the presence or absence of a Lewis acid co-catalyst, etc. The amount of the onium iodide, calculated as weight iodide, typically will be in the range of about 0.1 to 30 weight percent based on the total weight of the catalyst. Preferred loading levels fall in the range of about 0.5 up to 20 weight percent (same basis).
When present, the quantity of Lewis acid component of the catalyst compositions generally is in the range of about 0.01 to 30 weight percent, based on the total weight of the catalyst. The preferred quantity of the inorganic Lewis acid co-catalysts, e.g., titanium (IV) iodide, zirconium (IV) iodide, zinc iodide and tin (II) iodide, is in the range of about 0.02 up to 5.0 weight percent based on the total weight of the catalyst.
Another embodiment of the catalyst compositions provided by our invention comprise a support material having deposited thereon (i) about 0.1 to 30 weight percent of an organic onium iodide and (ii) about 0.01 to 30 weight percent of an organotin (IV) compound or organoantimony (V) halide, based on the total weight of the catalyst composition. These catalyst compositions preferably comprise:
(i) about 0.5 to 20 weight percent of a tetra-substituted ammonium and/or phosphonium iodide compound of formula (VII) and/or (VIII); and
(ii) about 0.02 to 20 weight percent of an organotin iodide containing a total of about 3 to 24 carbon atoms and having the formula
(R7)nxe2x80x94Snxe2x80x94I(4xe2x88x92n)
xe2x80x83wherein
each R7 substituent independently is selected from alkyl of up to about 12 carbon atoms, benzyl, phenyl or phenyl substituted with up to 3 substituents selected from lower alkyl, lower alkoxy or halogen; and
n is 2 or 3; on
(iii) a support material selected from silica, alumina, zinc oxide, titanium oxide, boron nitride and silicon carbide.
The supported catalysts described herein may be prepared by a variety of procedures as will be readily apparent to those skilled in the art. For example, the supported catalysts may be prepared by the steps comprising:
(a) impregnating a suitable support with a solution of one or more quaternary organic onium iodide compound and, optionally, one or more Lewis acids, and thereafter
(b) removing the solvent from the impregnated support.
Solvents contemplated for use in the impregnation step include polar solvents capable of substantially dissolving the quaternary organic onium iodide and the optionally employed Lewis acid. Such solvents include water, lower alcohols such as methanol, ethanol, isopropyl alcohol, and the like. Preferred solvents are those which can be easily removed by standard evaporative techniques once the desired impregnation has been carried out.
The volume of solvent required is 0.5 to 20.0 ml of solvent (plus quaternary organic onium iodide and, optional Lewis acid) per gram of support, the minimum volume of solvent being defined as that volume required to cover the catalyst support. The support and impregnation solution are agitated (typically by rotary tumbling) for 0.2 to 2.0 hrs at slightly elevated temperatures, e.g., 20 to 60xc2x0 C., to maximize interaction of support and catalytic components. The solvent is preferentially removed by rotary evaporation at reduced pressure at temperatures ranging from 40 to 100xc2x0 C., or alternatively by drying in a heated forced air oven, or further alternatively by spray-drying the catalyst solution on the support. After drying, the catalyst is ready to be loaded into a reactor.
Prior to contacting the catalyst with an epoxyalkene under isomerization conditions, the catalyst optionally may be subjected to pre-treatment conditions of time and temperature sufficient to activate said catalyst relative to non-pretreated catalyst. Typical pre-treatment conditions of time and temperature comprise a temperature at least as high as the melting point of said quaternary organic onium iodide, but no greater than 225xc2x0 C., for a time in the range of about 0.1 up to 10 hours.
The conditions of temperature and pressure and the space velocity employed in our novel isomerization process can vary considerably depending on various factors such as the activity of the catalyst used, the degree of conversion and/or selectivity desired, the gas hour space velocity employed, the mode of operation and the like. For example, the process may be carried out at a temperature in the range of about 60 to 225xc2x0 C. although temperatures of about 100 to 200xc2x0 C. are more typical. The total reaction pressure may be in the range of about 1.02 to 70 bar (absolute) with the preferred range being about 1.1 to 20 bar total pressure.
The gas hourly space velocity may be varied substantially, e.g., from about 1 to about 10,000, although our process normally is performed using gas hourly space velocities in the range of about 10 up to 5,000 (hrxe2x88x921). The epoxyalkene reactant may constitute up to 100% of the feed composition or may be fed along with an inert diluent wherein volume ratio of the epoxyalkene:inert diluent can vary over a wide range of values, typically within 1:100 to 4:1. Exemplary inert gas diluents include helium, argon, nitrogen, carbon dioxide, or hydrocarbons which are gaseous under reaction conditions. Preferably, the epoxyalkene concentration is in the range of about 5 up to 80 volume percent of the feed composition.
The isomerization process may be carried out using the catalysts described herein either in a supported or unsupported form. Thus, the supported catalysts may be utilized in fixed or fluidized beds using reactor configurations well-known to those skilled in the art.
When the catalyst is unsupported, it can be used at temperatures either below, at or above the melting point of the quaternary organic onium iodide salt. When the catalyst is at or above its melting point and exists as a substantially liquid phase, it is necessary to maintain the catalyst in a reactor volume such that the passage of the gaseous feed and product molecules is not restricted, yet contains the catalyst in the reactor volume. An up-flow reactor is suitable for this purpose since the gaseous feed maintains the catalyst in the appropriate position in the reactor, yet permits the passage of unreacted feed and reaction products through the liquid, or substantially liquid, phase catalyst and into the downstream refining/recycle apparatus. In an especially preferred mode of operation, the catalyst is in a vessel with a closed bottom and the feed is added through a gas dispersion apparatus below the level of the catalyst. The unreacted feed and reaction products can exit from the top of the reactor.
The unsupported catalyst system preferably is used in our process as a melt of an intimate mixture of one or more of the quaternary onium iodide compounds and, optionally, one or more of the Lewis acid co-catalysts described hereinabove. The onium iodide:co-catalyst weight ratio of the unsupported catalyst system can vary substantially, e.g. from 500:1 to 1:100, depending on the particular co-catalyst selected. The preferred onium iodide:co-catalyst weight ratios depend on whether the co-catalyst is (1) an organotin (IV) compound or an organoantimony (V) halide or (2) one of the other Lewis acids described herein above. Thus, for the unsupported catalyst systems containing an inorganic Lewis acid, such as titanium (IV) iodide, zirconium (IV) iodide, zinc iodide and tin (II) iodide, the preferred onium iodide:co-catalyst weight ratio is about 200:1 to 5:1 and for the organotin (IV) compounds and organoantimony (V) halides the preferred onium iodide:co-catalyst weight ratio is about 1:100 to 50:1. Particularly preferred unsupported catalyst systems comprise a mixture of one or more of the tetra-substituted ammonium or phosphonium iodide compounds described hereinabove and tin (II) iodide, zinc iodide or an organotin iodide.
The unsupported quaternary organic onium iodide and/or Lewis acid catalyst may be used with an inert organic solvent if desired to alter the reaction conditions and/or reactor configuration. The optional, inert organic solvent may be used, for example, to change the concentration of the quaternary organic onium iodide and/or the Lewis acid or to assist in heat and/or mass transfer characteristics of the catalytic process.
Thus, another embodiment of our invention comprises the isomerization of an epoxyalkene to the corresponding 2,5-dihydrofuran in the presence of a homogeneous catalyst solution. This embodiment may be carried out in the presence of one or more of the above-described organometallic compounds although reaction rates are relatively slow if an organic onium iodide is not included. Accordingly, the homogeneous catalyst solution preferably comprises a catalytic amount of (i) one or more of the above-described organometallic compounds and (ii) one or more of the above-described organic onium iodides in (iii) an inert organic solvent, i.e., a solvent which does not react with the xcex3,xcex4-epoxyalkene or xcex3,xcex4-epoxycycloalkene reactants or the 2,5-dihydrofuran products. Examples of the solvents which may be used include aliphatic and aromatic hydrocarbons such as heptane, toluene, specific or mixed xylenes, pseudocumene, and mesitylene; halogenated hydrocarbons such as chlorobenzene, 1,2-dichlorobenzene, and 1,1,2,2-tetrachloroethane; ketones such as cyclohexanone, 5-methyl-2-hexanone, and 2-heptanone; ethers such as 2,5-dihydrofuran, tetrahydrofuran, and bis(2-methoxyethyl)ether; esters such as isobutyl acetate; and tertiary amides such as N-methyl-2-pyrrolidinone, N-cyclohexyl-2-pyrrolidinone, N-ethyl-2-pyrrolidinone, and N,N-dimethylacetamide. Normally, for ease of separation, the solvent or mixture of solvents employed have boiling points at least 20xc2x0 C. above the boiling point of the 2,5-dihydrofuran product and the unsaturated aldehyde or ketone by-products.
The concentrations of the organometallic compound and the optional onium iodide in the inert, organic solvent can be varied substantially depending, for example, on the particular catalytically-effective components present, the design of the reactor system, etc. Typically, the concentration of the organometallic compound will be about 1 to 50 weight percent and the concentration of the onium iodide compound, when present, will be about 1 to 70 weight percent, both concentrations being based on the total weight of the catalyst solution. Normally, the mole ratio of onium iodide to organometallic compound is at least 1:1.
The preferred catalyst solutions comprise
(i) about 1 to 25 weight percent of an organotin iodide containing about a total of about 3 to 24 carbon atoms and having the formula
(R7)nxe2x80x94Snxe2x80x94I(4xe2x88x92n)
xe2x80x83wherein
each R7 substituent independently is selected from alkyl of up to about 8 carbon atoms, benzyl, phenyl or phenyl substituted with up to 3 substituents selected from lower alkyl, lower alkoxy or halogen; and
n is 1, 2, 3 or 4; and
(ii) about 1 to 25 weight percent of a tetra-substituted ammonium or phosphonium iodide of formula (VII) and/or (VIII); and
(iii) an inert organic solvent selected from hydrocarbons and chlorinated hydrocarbons having up to about 10 carbon atoms.
Toluene, mixed or specific xylene isomers, chlorobenzene, mixed or specific dichlorobenzene isomers, pseudocumene, and mesitylene are particularly preferred solvents.
The isomerization process may be carried out in the liquid phase using the catalyst solutions described hereinabove by contacting a xcex3,xcex4-epoxyalkene or xcex3,xcex4-epoxycycloalkene at a temperature of about 50 to 200xc2x0 C., preferably about 100 to 150xc2x0 C., depending on the solvent or mixture of solvents employed. The process may be carried out at atmospheric or super-atmospheric pressures, e.g., up to about 22 bar (absolute).
The process employing the catalyst solution may be carried out in a batch, semi-continuous or continuous mode of operation. For example, batch operation may comprise refluxing a mixture of the xcex3,xcex4-epoxyalkene and catalysts, e.g. tributyltin iodide and tetraheptylammonium iodide, in a solvent such as p-xylene for a time sufficient to convert essentially all the epoxide to the 2,5-dihydrofuran. The products are then separated by distillation from the mixture. The undistilled catalyst solution may be reused in a subsequent reaction.
The catalyst solution preferably is employed in a continuous mode of operation wherein a xcex3,xcex4-epoxyalkene or xcex3,xcex4-epoxycycloalkene is added to a recirculated catalyst solution which is then introduced into a continuous reactor. After isomerization, the reaction stream is fed to a distillation system for removal of product or products and recycle of the catalyst solution. Examples of continuous reactor designs in which the process can be performed are continuous stirred tank reactors and plug flow reactors.